The present invention relates to current source as it relates to an integrated circuit. More particularly, the present invention relates to current sources designed to provide rejection of high power supply voltages.
Portable and low power consumption electronic devices such as, e.g., laptop computers, personal digital assistants (PDAs), cellular telephones, and the like, are requiring less and less power to operate. This is due, in part, to the continued advancement in computer and electronic technology, with attention to the design and manufacturing of the components used in the construction of the low power consuming devices. These new low power devices have become more and more powerful such that they are now able to perform processing tasks previously associated with desktop and workstation computers.
However, the circuitry and components implemented in these low power consuming devices is of a design usually implemented in high power consumption devices such as, e.g., engineering workstations and desktop computer systems. In a low power consuming device, this amount of voltage would cause serious damage to the components and circuits contained therein.
Therefore, because the circuitry and components are not particularly well suited for implementation in low power consuming devices, modification of the existing components and circuits or entirely new designs are necessary. Additionally, it is advantageous to provide a current source that is relatively constant and highly insensitive to changes in supply.
One such component associated with most devices, whether high power consumption devices or low power consumption devices like those mentioned above, is the current source. The current source is an important component in circuits.
FIG. 1 shows a conventional current source 1000 commonly used in many of today""s circuits. Transistor 400 is the bias transistor and has an emitter coupled to resistor 200, a base coupled to voltage bias node 800, and a collector coupled to bias current output 600. Current flow through transistor 400 is indicated by arrow 401. Resistors 100 and 200 are coupled to the power source, Vdd, 900. Diode 300 has its anode side coupled to resistor 100 and its cathode side coupled to voltage bias node 800. The voltage bias node 800 is coupled to the base of transistor 400. Current source 500 is coupled to output node 800 and ground 700. Current flow at current source 500 is indicated by arrow 501.
Still referring to the circuit FIG. 1, it is known that the most dominant error source in circuit 1000 was that when the supply changed, transistor 4 reacted to that change. In this figure, when power supply 900 changes, the collector emitter voltage of transistor 400 will vary with the change in supply. The voltage at node 800 is established through resistor 100 and diode 300 to bias transistor 400. Because the power supply 900 may vary, and circuit 1000 is susceptible to change in the supply, this circuit does not provide the relative stability needed for use with low power consuming devices. This is reflected in the following equations.
Where Ic1=f(Vce1), Vbe=constant, Vt=kT/Q are given, and
where,
Ic1 is the collector current of transistor 400
Vbe is the base emitter voltage of transistor 400
Vt is the result of the diode equation from above
Vce1 is the collector emitter voltage of transistor 400
V1 is the load voltage at bias voltage 600
Va is the early voltage of transistor 400
Vdd is power supply 900
R1 is emitter resistor 100
R2 is emitter resistor 200
Is is the current source 500
Isat is the saturation current of diode 300 and
transistor 400
The collector current of transistor 400 is:
Ic1=Isat exp(Vbe/Vt)(1+Vce1/Va)xe2x80x83xe2x80x831)
where Vce=Vddxe2x88x92V1xe2x88x92(Ic1*R2)
Inserting Vce into equation 1, we obtain
Ic1=Isat exp(Vbe/Vt)[1+((Vddxe2x88x92V1xe2x88x92Ic1*R2)/Va)xe2x80x83xe2x80x832)
Assuming Va=20, V1=1V, and Vdd changed from 2 to 6 volts, current Ic will approximately change 20%.
Therefore, as shown in the equations above, current source 1000, designed originally for use in a high power consuming device, is not particularly well suited for implementation in a low power consuming device which requires a relatively stable current source. This is due, in part, to the fact that the current of traditional current sources has low power supply rejection because of early voltage. While advancement in BJT (bi-polar junction transistor) technology has been directed toward shallower 3unctions, which therefore has lower Va (early voltage) result, it is still susceptible to a change in supply. In attempt to provide a circuit more insensitive to a change in supply than circuit 1000, a cascode current device was introduced.
A cascode current device, historically, is a commonly used cure all to increase a circuit""s immunity to change in power supply voltage. A cascode current device is a series transistor, and, in the prior art FIG. 1, would be disposed between transistor 400 and bias current output 600. The cascode current device would have the effect of reducing the dependency of the circuit on power supply 900, Vdd, such that the reaction of transistor 400 to those changes in Vdd is reduced, thereby becoming more stable.
However, the cascode device is not without drawbacks. Because the cascode current device is a series transistor, and as such requires power, additional voltage must be added to the supply voltage to account for the voltage dropped across the cascode current device. Because the cascode current device requires the minimum power supply voltage to be increased, this is contradictory to the requirements of a low power consuming device or circuit.
Additionally, another drawback is that the cascode current device requires additional real estate within the component or device in which it will be implemented. This might not be possible due to the component""s or device""s limited amount of available physical space which could therefore require a complete redesign of the device or component.
Thus, a need exists for a current source circuit that can operate at low power supply voltage levels, such as those down to one volt. An additional need exists for a current source circuit that is highly immune to changes in power supply voltage levels. A need also exists for a current source circuit that can compensate for and reject high supply voltages. A further need exists for a current source circuit that requires negligible additional power for proper operation.
Accordingly, the present invention provides a bias current source for providing high power supply rejection. The present invention further provides a bias current source for use in low power consumption markets. The present invention additionally provides a highly stable bias current source.
The present invention provides a bias current source circuit with very high power supply rejection even with very low Va (early voltage). In one embodiment, the present invention is comprised of a bias current source circuit. The bias current source circuit is comprised of a primary current source coupled to a power supply, a secondary current source for biasing the primary current source and also coupled to the power supply, and a simple gain stage amplifier. In the present embodiment, the primary current source includes a first transistor having a first region coupled to a second resistor and a second transistor having a first region coupled to the second resistor. In the present embodiment, the gain stage amplifier includes a first input coupled to a second region of the first transistor. The gain stage amplifier further includes a second input coupled to a second region of the second transistor. The gain stage amplifier also includes an output coupled to the first regions of the first transistor and the second transistor.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.